1. Field of the Invention
This invention relates to an air vent apparatus for a water tube. More specifically, the invention relates to an air vent apparatus for a water tube, which is designed to be capable of preventing the occurrence of a water reservoir in the water tube by conveniently and easily venting air liberated in the water tube.
2. Description of the Related Art
In a thermal power plant, a nuclear power plant, and a combined power plant, seawater is often used as cooling water (circulating water) for use in a condenser.
As shown in FIG. 2, circulating water (seawater) is drawn from a water intake pit (sea) 2 by a circulating water pump 1, and the drawn circulating water (seawater) is fed to a condenser 4 via a circulating water tube 3a located on a water supply side. In the condenser 4, an exhaust gas sent from a steam turbine is cooled with the use of the circulating water (seawater) to cool steam in the exhaust gas for condensation. The circulating water (seawater), which has increased in temperature upon use in cooling, is released into a water release pit (sea) 5 through a circulating water tube 3b located on a water release side.
A shellfish removing device 6 is interposed in the circulating water tube 3a on the water supply side, while a condenser cleaning device 7 and a flow control valve 8 are interposed in the circulating water tube 3b on the water release side.
Iron is used as a tubular material for the circulating water tubes 3a, 3b, and a lining material such as a rubber material is applied to the inner peripheral surface of each of these tubes, for example, by coating. The provision of such a lining material prevents the corrosion of the circulating water tubes 3a, 3b by circulating water (seawater).
To prevent the deposition of shellfishes, etc. on the inner peripheral surfaces of the circulating water tubes 3a, 3b, moreover, the flow velocity of circulating water (seawater) flowing in the circulating water tubes 3a, 3b is set, for example, at a predetermined velocity of the order of 3 to 4 m/second.
Rendering the flow velocity lower than this predetermined velocity poses the problem of deposition of shellfishes on the inner peripheral surfaces of the circulating water tubes 3a, 3b, thereby decreasing the channel areas of these tubes or clogging these tubes. The flow velocity in excess of the predetermined velocity, reaching 5 m/second or higher, for example, causes the problem of peeling off the lining material coated on the inner peripheral surfaces of the circulating water tubes 3a, 3b. 
Thus, the flow velocity of circulating water (seawater) flowed in the circulating water tubes 3a, 3b is adjusted to the predetermined velocity (for example, 3 to 4 m/second).
The circulating water (seawater) utilized in cooling (heat exchange) in the condenser 4 has an increased temperature, and is thus in a state where air dissolved in seawater tends to be liberated. At a negative pressure, in particular, air tends to become liberated from seawater.
Air liberated from the seawater accumulates in the water box of the condenser 4. If the amount of the accumulating air increases beyond a certain level, the performance of the condenser 4 declines. Thus, the condenser 4 has hitherto been equipped with an air vent apparatus. If the amount of air accumulating in the water box reaches a certain value or higher, this accumulating air is discharged to the outside by the air vent apparatus (see Japanese Patent Application Laid-Open No. 1998-227581).
In the circulating water tube 3b on the water release side, circulating water (seawater), which is flowing, increases in temperature, making it easy to liberate air dissolved in seawater. If there are ups and downs in the piping route of the circulating water tube 3b on the water release side, the internal pressure of the circulating water tube 3b may become negative because of the hydraulic gradient. In the circulating water tube 3b on the water release side under such circumstances, air may be liberated from seawater to generate an air reservoir, in a negative pressure zone where the internal pressure becomes negative.
As shown in FIG. 3, for example, the height of the circulating water tube 3b disposed may be once raised, and then lowered, along the flowing direction of water (i.e., α-direction in FIG. 3). In this case, an air reservoir A may occur in a portion where the height of the circulating water tube 3b begins to be lowered (i.e., β portion in FIG. 3).
If such an air reservoir A occurs, the channel area for the circulating water decreases by the amount corresponding to the air reservoir A. As a result, the flow velocity of circulating water at this site increases, thereby causing a possibility for the peeling of the lining, or presenting the problem of an increased pressure loss.
Thus, there has been need for a piping plan according to which no air reservoir is formed halfway through the circulating water tube 3b. 
If there are ups and downs in the circulating water tube 3b, moreover, it has been necessary to check the hydraulic gradient, thereby making sure that the interior of the tube is not at a negative pressure, and work out a contrivance for avoiding a negative pressure.
An example of the contrivance for avoiding a negative pressure is that, as shown in FIG. 2, the flow control valve 8 is provided in a portion, as downstream as possible, of the circulating water tube 3b on the water release side, and its valve opening is properly adjusted, whereby the pressure of the entire channel of the circulating water tubes 3a, 3b is brought to a positive pressure so that no negative pressure occurs throughout the watercourse of the circulating water tubes 3a, 3b. 
Earlier technologies involve the following problems:
(1) Particularly when equipment is designed under a scrap-and-build approach, the ground area is often limited, so that there may be restrictions on the layout because of the effective use of or interference by existing equipment. In this case, planning of a piping route free from the formation of an air reservoir may be difficult.(2) It is necessary to check a hydraulic gradient in view of a piping plan, and reflect the results of checks in the piping plan. Thus, planning and designing require time and labor.(3) The flow control valve 8 for ensuring a pressure is installed in the circulating water tube 3b on the water release side and, accordingly, the necessity for raising the pump head of the circulating water pump 1 may arise. In detail, if it is attempted to render the pressure of the entire watercourse of the circulating water tubes 3a, 3b a positive pressure by providing the flow control valve 8 and appropriately adjusting its valve opening, the fluid resistance of the flow control valve 8 increases. To flow an appropriate amount of circulating water, therefore, there arises the need to raise the pump head of the circulating water pump 1.(4) If the air reservoir A occurs in the circulating water tube 3b as shown in FIG. 3, the channel area for circulating water decreases by the amount corresponding to the air reservoir A. As a result, the flow velocity of circulating water at this site increases, thereby causing a possibility for the peeling of the lining, or presenting the problem of an increased pressure loss.
The present invention has been accomplished in light of the above-described problems with the earlier technologies. It is an object of the invention to provide an air vent apparatus which can vent air from a portion of a water tube, such as a circulating water tube, where an air reservoir tends to occur.